Field of the Invention
The present invention relates to processes for destroying organic binders in ceramic components during the firing process.
Discussion of the Background
Many ceramic objects are made by first mixing organic binders and other organic additives with very fine raw ceramic powders. This mixture is then cast or compressed into a desired shape and then subjected to a high temperature firing procedure.
The main function of the organic binder is to provide the "green" ceramic component with strength and rigidity. These properties are important because they permit the formed components to withstand moderate handling and transportation requirements which the parts may be subjected to between the forming and firing process.
The primary function of the firing process is to sinter the fine compacted raw ceramic powders together, after binder burn out, and thus permanently bond the aggregate mass of fine powder into a compact, dense, and rigid single component item. Typically, the firing process is carried out in an open belt furnace, box furnace, or kiln. Consequently, during the firing process, the ceramic components are exposed to a stagnant or flowing oxygen-rich atmosphere (in most cases, air).
The oxygen-rich atmosphere helps to burn out the organic binders (when they are used) and also facilitates the removal of gaseous products and smoke produced from binder decomposition and pyrolysis. To prevent an explosive reaction between these decomposition products and the oxygen-rich atmosphere, heating rates are carefully controlled.
By this process, combustible gaseous and solid decomposition products are produced so slowly that the influx of fresh air is always great enough to prevent an explosive reaction, the concentration of reactive gases and/or solids being always maintained below the lower explosive limit by dilution. However, this severely limits product production rates because it requires the use of very slow heating rates.
One method of overcoming this disadvantage in production rates is to fire the binder containing products in a chemically inert gas atmosphere (i.e. nitrogen, argon, helium, etc.) instead of air. Then, much more rapid heating rates can be used during the heating stages which precede sintering.
When an inert gas atmosphere is used in this type of process, combustible gases and solids cannot explode because a strong oxidizing agent is not present. Although this technique is very safe, its use can lead to deposits of residual carbon within the sintered ceramic component. In general, residual carbon deposits, within the fired and sintered ceramic components, are undesirable.
There is thus a need for a more efficient process for destroying organic binders in ceramic components during the firing process. Such a process would ideally safely destroy the organic binders and would not require the use of very slow heating rates.